In an effort to remain competitive, engine manufacturers are continuously seeking ways to improve the efficiency and reliability of their engine products without compromising performance. Through research and innovation, manufacturers are continuously attempting to reduce manufacturing costs, yet provide the customer with a reliable and efficient product that meets or exceeds their needs. A known technique for achieving greater efficiency, especially in engines used in over-the-road vehicles, is to reduce the weight of such engines. Such weight reduction can lead to greater fuel efficiency, reduced tire wear, and other reduced costs associated with manufacture and use of the engine product.
The camshaft of an internal combustion engine has evolved through the years to meet ever increasing performance requirements, e.g. increased stress tolerances, need for longer durability, and cost effective manufacture. In certain types of engines, such as diesel engines used in over-the-road commercial trucks, manufacturers have increased injection pressures to improve the performance, efficiency and lowered emissions to meet governmentally mandated standards. However, these high injection pressures have significantly increased stress tolerances and torsional loads on such engine cam shafts. Increasing the camshaft's diameter is one way to meet such increased demand. One problem associated with using a large diameter camshaft, however, is the amount of weight it adds to the engine. Hence, at least some of the benefits associated with a camshaft of unusually large diameter could be lost unless its weight is minimized.
Another problem faced by engineers in the engine industry is designing an engine that provides an adequate amount of lubricant to the camshaft and camshaft bearing journals in order to cool these parts, reduce undesired friction and minimize wear during engine operation. If any of these factors are not met, the engine could suffer substantial damage and possibly engine failure.
Certain engine manufacturers have attempted to develop hollow camshafts to reduce the weight of the engine while trying to provide adequate lubrication to the camshaft journal bearings. For example, U.S. Pat. No. 4,957,079 to Nakatani et al. discloses an exhaust overhead camshaft formed with an axial oil passage extending along substantially its entire length and communicating with radial oil passages formed in the camshaft bearing journals. An oil passage extends upward from midway of a laterally extending oil passage and opens to an annular groove of a plain split thrust bearing for the exhaust overhead camshaft. The engine lubrication oil flows through the oil passage and into the annular groove of the plain split thrust bearing for the exhaust overhead camshaft, to oil the thrust collars. The lubricating oil passing up to the thrust collars further flows, through the radial oil passages formed in the thrust collars, into the axial oil passage in the camshaft. The radial oil passages formed in the camshaft bearing journals of the camshaft allow the lubricating oil to flow in the axial oil passage to lubricate the bearings of the camshaft.
The '079 Nakatani patent discloses only one inlet for lubricant to flow into the axial oil passage of the camshaft which limits the volume and distribution of lubricant to the camshaft bearing journals during engine operation. In addition, if the one inlet of Nakatani becomes clogged, no lubricant would be available for the camshaft bearing journals potentially causing severe engine problems. In addition, the structural design of the Nakatani camshaft does not allow for even distribution of lubricant from the engine head to the camshaft journal bearings since lubricant is introduced only at one end of the camshaft. As stated above, it is imperative that lubricant is allowed to enter into the camshaft unimpeded to prevent any clogging or other undesirable event which could impair fluid communication between engine parts and impair adequate lubrication of critical engine parts.
By creating a hollow camshaft structure including a hollow shell with radial holes formed therein, a manufacturing engineer must consider torsional and other load influences on the camshaft body during engine operation. A hollow camshaft used in a large, heavy duty engine environment must be able to withstand high injection pressures and other stress-related forces which can over stress or even break the camshaft. Therefore, the hollow camshaft must be formed in a way that reduces the impact of torsional loads exerted on the camshaft during engine operation while providing adequate lubrication to the camshaft journal bearings. The '079 patent does not suggest the desirability of maximizing the volume and selecting the shape of the hollow interior to reduce thereby the weight of the camshaft while also producing adequate strength and other operating characteristics as discussed above.
One reference which focuses on this problem is U.S. Pat. No. 4,072,448 to Loyd, Jr. which discloses holes formed in a camshaft body to allow lubricant to flow therethrough. Each of the holes are formed spaced apart in different planes in the camshaft body. The formation of the holes in this manner improves the load characteristics of a hollow camshaft. However, the structural design of the Loyd camshaft does not insure adequate fluid communication and distribution to the camshaft journal bearings and other critical areas of the camshaft.
It is evident, based on the references discussed above, no hollow camshaft has been developed which provides effective fluid communication between the engine cylinder head, camshaft and camshaft journal bearings while operating under high injector pressures and torsional loads.